Witches Broom on Blue Palo Verde

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Witches broom symptoms on palo verde were first described over 50 years ago, but their cause was unknown. Our research results have identified the presence of a previously unknown multi-segmented, single-stranded RNA plant virus, in the genus Emaravirus, in shoots, leaves, flowers, seeds, and suckers from blue palo verde trees that were affected by broom.

The presence of the single-stranded RNA virus was not detected in the same tissues tested from plants showing no symptoms of broom disease. This plant virus associated with broom-affected palo verde trees had not been discovered until now. About 60 percent of the viral genome sequence is similar to another plant virus named High Plains wheat mosaic virus.

Other emaravirus species are known that infect European mountain ash (Sorbus aucuparia), fig (Ficus carica), pigeonpea (Cajanus cajan), raspberry (Rubus spp.), and rose (Rosa multiflora). The symptoms characteristic of emaravirus infection in these species are blotching, chlorotic ringspots, mottling of leaves, smaller than normal or stunted leaves, and densely clustered, multiple shoots.

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All emaraviruses discovered so far have been found to be transmitted by an eriophyid mite, making it the only known ‘biological’ vector. These arthropods are often host specific, meaning they populate only one genus or possibly a single species of plant. The presence of the eriophyid mite Aculus cercidi (Keifer, 1965) has been associated with broom symptoms in palo verde for some time, and it was suspected to cause feeding damage that resulted in the development of broom symptoms. However, until now, no causal relationship had been established between the eriophyid mite and the broom symptoms in palo verde tree.

Based on several years of collecting insects and arthropods from broom infested and healthy, non-symptomatic trees, we found consistently high populations of A. cercidi infesting trees with broom symptoms, while mites were in low abundance or rarely found in apparently healthy, non-symptomatic trees. Confirming the presence of the emaravirus in mites collected from broom infected trees provided further evidence of mites as the vector in transmitting the witches broom disease. This strong association between the presence of the emaravirus in broom of blue palo verde trees that were heavily infested with the suspected eriophyid vector, the absence of both virus and vector on non- symptomatic trees, and the presence of the virus in the eriophyid mites, strongly support a causal relationship between emaravirus and the witches broom disease of blue palo verde.

Eriophyid mites are sometimes confused with spider mites. Eriophyid mites are wedge-shaped, very small, and require an approximately 20x magnification hand lens to see on a plant and at least 80x magnification to observe the different life stages more closely under a microscope. Identification of eriophyid mite species requires an image taken with an electron microscope and the assistance of a mite expert; mite samples we collected have been identified as A. cercidi using these tools.

There is no cure for witches’ broom in blue palo verde. Our current project goals are to assist nurseries in producing virus-free and broom-free blue palo verde trees to reduce the source of virus in trees that are transplanted into landscapes. We are testing whether seeds from blue palo verde that is collected from broom affected shoots will result in seedlings and trees that are infected with the virus. We are also testing whether the use of miticides to control eriophyid mites on tree seedlings will prevent witches broom disease in the nursery.

Cutting out the shoots with broom symptoms in small or larger trees has not been a consistent cure for witches’ broom, as new shoots with broom symptoms usually regrow within a short period of time. In some cases broom regrowth did not occur immediately, but emerged again later.

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